Liquid Distributor – An Important Tower Internal (Part 2)

Liquid Distributor

DESIGN ASPECTS OF LIQUID DISTRIBUTOR

In this section, we will discuss the parameters that need to be considered while designing a high-performance liquid distributor.

Drip Point Density:

The drip point density is the specific number of final distribution points per m2 of cross-section area on top of the packing bed. For systems in contact with low to medium liquid load, minimum outlet diameters and minimum liquid levels result in maximum achievable density.

Placement of the feed points:

Liquid Distributor
The quality of distribution can be expressed as a value. A distributor with a free overflow, (q = 40%), only saturates the line between the trough with water. A distributor with a base hole (q=60%) generates a concentrated area below the trough. A distributor with a side hole (q=80%) and pipe guidance system creates the best distribution pattern, where empty spaces are at a minimum.

Internal distribution hydraulics

Liquid Distributor
A good liquid distributor releases water from its outlets in a uniform flow.

Distributors with a small flow cross-section, therefore high velocity, generates a pressure drop across the length of the individual distributor, with effects on the outflow volume at the feed points. The latter described distributors are less expensive. Each type of distributor operates within a certain range of maximum flow velocity. While determining the flow velocity for a distributor, the inlet and outlet cross-section should be carefully dimensioned

Distance to the bed

The minimum distance to the bed is determined by the gas flow and the maximum distance by the height which the liquid drops and its impact momentum on the packing. Normal distance range between 80mm to 250 mm. In most cases, distances ranging from 120 to 160 mm provide trouble-free operation.

As the distance from the bed increases, the speed of the falling liquid jet also increases and the impact momentum on the bed is so high that a significant amount of fine splash droplets is generated. These droplets may be carried away by the gas flow. Avoidable entrainment problem may occur through a pressure increase in the droplet separator or through its flooding.

Orientation to the packing

The upper section of the bed does not achieve its full efficiency either with random or structured packings. The liquid must first spread out radially to be able to develop an effective area typical for actual load and systems. As a rule of thumb, a depth of 0.25m is assumed for random fills and the depth of a layer for structured packing.

For structured packing, attention must be paid to the orientation of the distributor to the direction of the packing sheets. To ensure that liquid is applied to as many gaps in between the foils as possible in the first layer a distributor orientation of 45 degrees to the sheet is recommended.

Free gas cross-section

Liquid distributors always present a resistance that generates a pressure loss for the gas flow. The free gas cross-section should, therefore, be designed to be as large as possible. This is often in conflict with the distribution quality.

Importance of flow dynamics

Flow dynamics provides a way of understanding parameters that directly impact liquid flow. As a result, having in-depth knowledge of flow dynamics is important for the efficiency, performance and design of liquid distributors. Here are the different aspects that flow dynamics helps with:

  • Facilitating uniform distribution: A proper understanding of flow dynamics can ensure that the design of a liquid distributor promotes uniform distribution of liquid between various phases.
  • Monitoring pressure drops: Since there is a close relationship between flow and pressure, a decrease in flow rate can signal a drop in pressure. As a result, flow dynamics can help with monitoring pressure, making it possible to determine a potentially high pressure drop.
  • Preventing backflow: Backflow causes water to flow back into the distribution system, increasing the chances of contamination. An understanding of flow dynamics can aid in ensuring the liquids flow in a single, intended direction.
  • Optimizing flow rates: Flow dynamics can provide insights into the volume and rate at which fluid flows through one given point at a specific period. Understanding flow rates is essential for numerous applications, including optimizing designs and processes to increase a system’s efficiency and effectiveness.
  • Avoiding dead zones: Understanding flow dynamics allows for the identification of improper mixing or stagnant flow areas, which can cause dead zones within the distribution systems. This knowledge can help engineers avoid dead zones by modifying the liquid distributors’ design for better mixing of fluids and improved, stabilized flows.
  • Minimizing erosion and corrosion: Flow dynamics provides a means for understanding and determining flow patterns, rates and force. As such, this knowledge can help reduce erosion and corrosion of liquid distributors by preventing higher flow velocities and turbulent conditions.
  • Distributing bubble columns: In the case of bubble column reactors, a precise knowledge of flow dynamics is required to efficiently design and operate these systems, especially where the interaction of gas and liquid is involved.

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MACH Engineering is experienced with liquid distributors and other tower internals. Contact us for more information regarding our expertise or for more information.

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